It has long been known that amylosic films may be cast on a surface from a solution of amylosic materials in a solvent, the resultant film being peeled from the surface. However, that method is unsuitable for mass production of amylosic films or for the production of shapes other than films.
In the 1960's various attempts were made to provide more practical methods of manufacture of amylosic films.
U.S. Pat. No. 3,117,014 describes a method of manufacture of shaped articles by forming a hot melt from a mixture of amylaceous material, plasticizer and water. It was shown that plastic flow increased with water content and that it was necessary to retain moisture during extrusion using an unvented extruder.
U.S. Pat. No. 3,243,308 showed that strong flexible edible films could be made by confining amylosic mixtures at high temperatures and under super atmospheric pressures prior to extrusion.
GB Patent No. 965,349 showed that it was advantageous to add from 10% to 50% by weight of water to the amylosic substance prior to extrusion and subsequently to heat and stretch the extruded film.
Notwithstanding the above developments in the early 1960's, and notwithstanding the low cost of amylosic raw materials, the resulting materials have not found widespread acceptance and the extrusion of starch hot melts has not been adopted commercially to any significant extent.
One reason is that although amylose films prepared by the above methods have some useful properties they lack others, in particular mechanical strength and sufficient ability to stretch. The films tend to be hygroscopic and if desired to have a shelf life require to be dried and then lacquered by brush, spray or dipping which is impractical for mass production.
A second reason is that amylose films produced to date have been unsuitable for lamination with other polymers with the result that amylose films have been unable to compete in terms of versatility, or properties with modern co-extruded laminated plastics films in which a plurality of layers each of different polymeric composition, and each layer selected to contribute specific properties, are purpose designed for particular end uses.
Because the elongation before failure of known amylosic films has typically been from about 4% to 20%, the films have not been suitable for use with techniques such as blow moulding, and have been of limited utility in other applications where a greater capacity for elongation is desired.